Those came into my last aquarium from some LR as well and soon they kind of took over their own side of the tank. They spread fast and got a good size bigger and were not that attractive. The guy at the LFS said I should kill them because they will sting what is around them and said I would not want any corals or anemones nearby. The funny this is, I just got some LR for my new tank today and One of those just appeared on a piece. I've been looking around for the best way to kill it. Maybe they are not what I'm thinking of but they looking very very close.
Oh I see MR. X just answered before me. Good to know, is their a chance my crab would eat it?